Orthodontics is a specialized dental practice concerned with the movement of teeth to achieve an effective occlusion (the proper mating of the upper and lower teeth), and to provide a pleasing facial contour and appearance of the teeth. Tooth movement is accomplished by applying a force to the tooth in the direction of desired movement. A relatively long-term application of corrective force of the proper level will cause the tooth root to move within the supporting bone (the upper and lower jawbones, respectively the maxilla and mandible) to enable tipping, rotation, translation, and other tooth movements needed to align maloccluded dental arches.
Corrective force is provided by the restoring force exerted by a stressed or activated elastic element such as flexible metal wire, a metal spring, or a band such as a "rubber" band made of a material compatible with the mouth environment. The most common and useful elastic element in modern orthodontics is a metal arch wire of roughly U-shape to conform to the array of teeth in each dental arch. The arch wire is coupled to the teeth by slotted orthodontic brackets secured to the teeth either by direct adhesive attachment, or by being fastened to a tooth band fitted over and cemented to the teeth. The arch wire is distorted or flexed when fitted into the bracket slots, and the resulting restoring force is exerted through the brackets on the teeth to urge them into proper alignment.
Critical factors in orthodontic treatment include the selection and placement of orthodontic brackets, and the contouring of the arch wire. The combination of these factors determines the direction and level of corrective force applied to each tooth. Control of direction is essential to insure that each tooth is urged toward a position of proper alignment. Control of force level is equally important, both for patient comfort, and because excessive force will interfere with and impede the bone resorption process which enables the individual tooth roots to move slowly within the supporting bone.
Control of corrective force is complicated by the fact that the optimum force differs from tooth to tooth (depending on the size and engagement of the root structure), and also somewhat with the direction of desired movement. This factor has led to a somewhat serial approach to movement of individual groups of adjacent teeth, and overall treatment time is relatively long. Any reduction in overall treatment time is desirable out of consideration for patient comfort and appearance, and because in some cases it can reduce the expense of treatment.
Perhaps the most common malocclusion problems diagnosed by orthodontists prior to corrective treatment involve crowded and irregularly positioned anterior or front teeth (the central and lateral incisors) in either or both arches, and a closed bite or so-called Class II malocclusion where the upper maxillary front teeth are excessively far forward of the mating lower mandibular front teeth. Both problems interfere with chewing function and cosmetic appearance of the teeth, and the closed-bite factor is often manifested as an undesirable chin contour.
Preliminary alignment or "unraveling" of the reltively small anterior teeth requires application of light corrective force, whereas initial correction of a closed bite requires heavier forces to reposition the more massive posterior teeth (molars and bicuspids) which have larger root structures. With known arch wires, these demands are incompatible in that a small and limber arch wire is needed for bracket engagement on the badly misaligned front teeth, whereas a heavier and more rigid wire is needed for correction of the closed bite.
The result is that initial alignment is typically done in several serial-order stages, using a light wire for anterior alignment, and a heavier wire for subsequent posterior movement. This approach lengthens overall treatment time to the detriment of the patient. Perhaps the best prior-art solution to the problem is provided in the Begg light-wire technique which in effect decreases the stiffness of the anterior part of the arch wire by forming a series of stress-relieving loops in the wire portion spanning the front teeth. Such looped arch wires have shortcomings, however, in that it is difficult to avoid loop impingement on gingival or lip tissue, more frequent arch-wire replacement is required as the treatment program progresses, and the loops interfere with oral hygiene and gum massage during brushing by the patient.
The arch wire of this invention overcomes these problems by providing segments of differing stiffness or flexural rigidity along the length of the wire. The individual segments are united to form a fuctionally integral arch wire which enables control of and application of different forces to the different groups of teeth undergoing treatment. This enables preliminary treatment to proceed in simultaneously executed parallel stages for a significant reduction in overall treatment time. The new arch wire is useful with both light-wire and conventional edgewise orthodontic brackets, and can be used as a conventionally contoured wire, or with brackets configured for newer straight-wire or lingual techniques.